FIG. 1 shows a conventional power converter apparatus of the type described. Designated at 1 is a DC power supply, and 2 a three-phase bridge circuit connected through a chopper thyristor 3 between power supply terminals P, N of the DC power supply 1. The bridge circuit 2 comprises a thyrister bridge circuit comprising thyristors 2.sub.UP, 2.sub.UN, thyristors 2.sub.VP, 2.sub.VN, thyristors 2.sub.WP, 2.sub.WN, and diodes 5.sub.UP -5.sub.WN coupled respectively to the thyristors in anti-parallel connection. By controlling the thyristors for conduction in a predetermined order, desired three-phase alternating currents can be generated from output terminals U, V, W. The DC voltage applied across input terminals A, B of the bridge circuit 2 can be controlled by controlling the ratio of conduction time of the chopper thyristor 3.
To the chopper thyristor 3 for the inverter circuit of the foregoing construction is coupled a chopper feedback diode 4 in anti-parallel connection. The inverter feedback diodes 5.sub.UP, 5.sub.UN, 5.sub.VP, 5.sub.VN, 5.sub.WP, 5.sub.WN are coupled respectively to the thryristors of the inverter circuit 2 in anti-parallel connection. A series-connected circuit composed of a commutation capacitor 6, an inverter commutation reactor 7, and a commutation thyristor 8 energizable when reverse-biasing the thyristors of the bridge circuit 2 is connected in parallel between the input terminals A, B of the bridge circuit 2. Another series-connected circuit composed of a commutation thyristor 9 energizable when reverse-biasing the chopper thyristor 3 and a chopper commutation reactor 10 is connected between the terminal of the chopper feedback diode 4 which is coupled to the DC power supply and the negative terminal as shown of the commutation capacitor 6. The series-connected circuits serve as commutation circuits for the inverter circuit 2 and the chopper thyristor 3.
Commutation operation of the arrangement of FIG. 1 will be described with reference to FIGS. 2(a) and (b) which show the waveforms of a voltage Vc and a current ic for the commutation capacitor 6. The voltage Vc (FIG. 2(a)) charged across the commutation capacitor 6 with the polarity as shown in FIG. 1 is discharged by triggering the thyristor 9 through a closed loop composed of the capacitor 6, the feedback diode 4, the thyristor 9, the reactor 10 and the capacitor 6. Then, the polarity of the voltage Vc (FIG. 2(a)) across the capacitor 6 is reversed. At this time, a pulse current ic.sup.+ (FIG. 2(b)) of a substantially sinusoidal waveform flows from the capacitor 6 to reverse-bias the chopper thyristor 3, which is extinguished.
Then, by triggering the thyristor 8, the voltage charged across the capacitor 6 in the opposite polarity by the pulse current ic.sup.+ is discharged through a closed loop composed of the capacitor 6, the reactor 7, the thyristor 8 and the feedback diodes in the inverter circuit 2, and the capacitor 6. The capacitor 6 is supplied with a negative pulse current ic.sup.- (FIG. 2(b)) of a substantially sinusoidal waveform. At this time, the thyristors 2.sub.UP -2.sub.WN in the inverter circuit 2 are all extinguished for forced communication.
Assuming that each of the inductances of the reactors 7, 10 is L, and the electrostatic capacitance of the capacitor is C, the pulse current has a crest value Ip and a half period interval tw as follows: ##EQU1## It is desirable that the circuit arrangement be designed for the crest value Ip and the half period time tw to meet the following relationships: ##EQU2## where IL is the crest value of a load current to be commutated, and tc is the reverse-biasing time. Therefore, the electrostatic capacitance C of the commutation capacitor 6 and the inductances L of the commutation reactors 7, 10 are required to have values defined by the following expressions: ##EQU3##
In the arrangement of FIG. 1, the commutation capacitor and the commutation reactors in the power converter apparatus greatly affect the dimension and cost of the apparatus as compared with the other semiconductor devices, a condition which constitutes a bottleneck in fabricating power converter apparatus of thyristor inverters. For the above reason, therefore, transistor inverters are more relied upon for power converter apparatus having a smaller capacity of several kW or less.
It is an object of the present invention to provide a commutation circuit which is simpler than the foregoing conventional arrangement.